Add docs page about environment variables

[gregdenton]

As we've built out more configuration by environment variables, we need to update the docs to explain what functionality is out there. Examples: $CUE_FRAME_LOG_DIR $CUESUBMIT_CONFIG_FILE

[sharifsalah]

Is there a straight forward way to identify a list of the available variable names for users? The two examples you listed are in quite different paths in the repo.

[gregdenton]

Not really right now. Part of this issue should really be to simply catalog and organize these types of environment variable and configuration settings. I can take a pass through that early next week.